The present invention relates to process devices. More specifically, the present invention relates to field-mounted process control and measurement devices.
Process devices are used to measure and control industrial processes such as the refining of petrochemicals, the processing of food, and a number of other processes. Process measurement devices include process variable transmitters, which measure a process variable such as pressure or temperature and communicate the measured variable to a process controller. Another type of process device is an actuator, such as a valve controller or the like. Generally, process control is accomplished using a combination of transmitters, actuators, and a process controller that communicate across a process control loop. Both types of process devices interact with the physical process through process interface elements. Process interface elements are devices which relate electrical signals to physical process conditions, and include devices such as sensors, limit switches, valve controllers, heaters, motor controllers, and a number of other devices.
The process controller is typically a microcomputer located in a control room away from the process. The controller can receive process information from the process measurement devices and apply a suitable control signal to one or more process control devices to influence the process and thereby control it.
In order to couple to the process, transmitters and actuators are generally mounted near the process in the field. Such physical proximity can subject the process devices to an array of environmental challenges. For example, process devices are often subjected to temperature extremes, vibration, corrosive and/or flammable environments, and electrical noise. In order to withstand such conditions, process devices are designed specifically for “field-mounting.” Such field-mounted devices utilize robust enclosures, which can be designed to be explosion-proof. Further, field-mounted process devices can also be designed with circuitry that is said to be “intrinsically safe”, which means that even under fault conditions, the circuitry will generally not contain enough electrical energy to generate a spark. Further still, electrical isolation techniques are usually employed to reduce the effects of electrical noise. These are just a few examples of design considerations, which distinguish field-mounted process devices from other devices, which measure sensor characteristics and provide data indicative of such characteristics.
Aside from the environmental considerations listed above, another challenge for field-mounted devices is that of wiring. Since process devices are located near the process far from the control room, long wire runs are often required to couple such devices to the control room. These long runs are costly to install and difficult to maintain.
One way to reduce the requisite wiring is by using two-wire process devices. These devices couple to the control room using a two-wire process control loop. Two-wire devices receive power from the process control loop, and communicate over the process control loop in a manner that is generally unaffected by the provision of power to the process device. Techniques for communicating over two-wires include 4–20 mA signaling, the Highway Addressable Remote Transducer (HART®) Protocol, FOUNDATION™ Fieldbus, and others. Although two-wire process control systems provide wiring simplification, such systems provide a limited amount of electrical power to connected devices. For example, a device that communicates in accordance with 4–20 mA signaling must draw no more than 4 mA otherwise the device's current consumption would affect the process variable. The frugal power budget of two-wire process devices has traditionally limited the functionality that could be provided.
While the development of field devices that communicate digitally over a process communication loop has marked a significant advance in the art of process industry measurement and control, there remains a significant difficulty. Specifically, many installations of field-mounted process devices have tens if not hundreds of various process devices coupled to the process at various locations and interacting with the process under the control of one or more controllers.
In the not too distance past, all field devices were analog-based field devices. Thus, at one point, the process industry applications had, or still have, a significant investment in a vast array of analog-based process field devices. Even in the advent of significantly advanced digital devices, such as those that communicate in accordance with the FOUNDATION™ Fieldbus process communication protocol, owners of analog installations are somewhat frustrated from simply adopting the new technology due to the time and investment involved in switching over their entire installation to the new technology.
There exists a significant need in the art to provide a device or system that facilitates implementation of digital process control and measurement technology without requiring an owner of such an installation to completely adapt their system to the new technology.